1. Field of the Invention
The invention disclosed in this specification relates to a thin film transistor and a fabrication method thereof and more particularly to a circuit and a device constructed by using a thin film transistor.
2. Description of Related Art
Hitherto, there has been known a thin film transistor (hereinafter referred to as a TFT or the like) constructed by forming a thin film semiconductor or a silicon semiconductor film in particular on a substrate.
While such a TFT is used in various integrated circuits, it is often used in an active matrix type liquid crystal display in particular. The active matrix type liquid crystal display has a structure in which the TFT is disposed as a switching device in each pixel electrode arranged in a matrix. There has been also known a liquid crystal display in which not only the matrix circuit but also a peripheral driving circuit thereof is constructed by TFTs (which is called a peripheral driving circuit integrated display).
The TFT is also used in various integrated circuits and multi-layered integrated circuit (three-dimensional IC).
It is convenient to use an amorphous silicon film which is formed by means of chemical vapor deposition such as plasma CVD as a silicon film used in the TFT. It may be said that this technology has been almost established.
However, the TFT using the amorphous silicon film has an electrical characteristic which is far lower than that using a single crystal semiconductor used in general semiconductor integrated circuits. Therefore, it is the present situation that it is used only in the limited uses such as a switching device of the active matrix circuit.
As a technological trend of the future, it is required to realize an arrangement in which the active matrix circuit, the peripheral driving circuit, an image processing circuit, an oscillation circuit and the like are integrated on one and the same substrate.
A crystal silicon film may be used, instead of the amorphous silicon film, in order to improve the characteristic of the TFT using the amorphous silicon film. A silicon film having a crystallinity beside single crystal silicon is called poly-crystal silicon, poly-silicon, microcrystal silicon and the like.
Such a silicon film having the crystallinity may be obtained by forming the amorphous silicon film at first and then by crystallizing it by heating (annealing). This method is called solid phase growth because the amorphous state changes to the crystal state while keeping the solid state.
However, there has been a problem that the solid phase growth of silicon requires more than 600.degree. C. of heating temperature and more than 20 hours of heating time and it is difficult to use a low cost glass substrate as a substrate.
For example, the Corning 7059.RTM. glass used for the active matrix type liquid crystal display has a glass strain point of 593.degree. C. and there is a problem in performing the annealing at 600.degree. C. or more when the increased area of the substrate is taken into consideration.
There has been also another problem in terms of productivity that it takes more than 20 hours for the heat treatment for crystallization.
In order to solve such problems, the inventors had developed a technology which allows the crystallization to be achieved at 550.degree. C. in about 4 hours of treatment time by depositing a trace amount of a certain kind of metal element such as nickel and palladium on the surface of the amorphous silicon film and then by heating it (Japanese Patent Laid-Open No. 6-244103, the disclosure thereof being incorporated herein by reference).
It is possible to obtain a silicon film having a better crystallinity when it is annealed at 600.degree. C. for 4 hours.
This technology allows a crystal silicon film having a large area to be obtained on a low cost glass substrate with a high productivity.
As methods for introducing such a trace amount of metal element (metal element which promotes the crystallization), there are methods of depositing a coating film of the metal element or its compound by means of sputtering as disclosed in Japanese Patent Laid-Open No. 6-244104, of forming a coating film of the metal element or its compound by means of spin coating or the like as disclosed in Japanese Patent Laid-Open No. 7-130652 and of forming a coating film by decomposing gas containing the metal element by means of thermal decomposition, plasma decomposition or the like as disclosed in Japanese Patent Laid-Open No. 7-335548. The disclosure of these Laid-Opens are incorporated herein by reference.
There is also a method of selectively introducing the metal element to a specific part and of then widening the growth of crystal from the part where the metal element has been introduced to the peripheral part (lateral growth method). The crystal silicon obtained by such a method has an oriented crystal structure and shows very excellent characteristics in response to the orientation.
The methods for fabricating the crystal silicon film by using a certain kind of metal element, e.g. nickel, is very excellent as described above. However, it has been found that there are problems when a TFT is fabricated by using such a crystal silicon film that its device characteristic varies and its reliability is low.
Accordingly, it is an object of the invention disclosed in the present specification to provide a technology which allows a TFT whose device characteristic varies less to be obtained in fabricating the TFT by using the crystal silicon film obtained by using metal element.